1.优化代码结构，抽象预览接口服务

2.新增更多图片预览格式支持

jodconverter-web/src/main/java/cn/keking/service/impl/PdfFilePreviewImpl.java

@@ -0,0 +1,19 @@
+package cn.keking.service.impl;
+
+import cn.keking.service.FilePreview;
+import org.springframework.stereotype.Service;
+import org.springframework.ui.Model;
+
+/**
+ * Created by kl on 2018/1/17.
+ * Content :处理pdf文件
+ */
+@Service
+public class PdfFilePreviewImpl implements FilePreview{
+
+    @Override
+    public String filePreviewHandle(String url, Model model) {
+        model.addAttribute("pdfUrl", url);
+        return "pdf";
+    }
+}